Assembly of mitochondrial ribosomes requires the transport of ribosomal proteins from their site of synthesis in the cytoplasm into mitochondria and the synthesis of ribosomal RNA's on m-DNA as a precursor molecule which is cleaved and methylated. It is proposed to study the control of this assembly process making use of a mutant Neurospora strain (Mi-1-poky) which shows a defect in assembly of small m-ribosomal subunits. The defect is apparently linked to mitochondrial- DNA. The experiments will invole a study of the control of enzymatic methylations within mitochondria, a search for nascent and maturing ribosomal subunits, and an investigation of how post-transcriptional modification of proteins may affect their transport into and function within mitochondria.